If/Then, a new musical about a city planner starting over in New York City, puts a light-up map of the New York City subway system in the floor of the stage. And if you can’t recognize it from its shape alone, when all the lines are illuminated in white (they get individual colors later), then this show is probably not for you. Following the precedent of venerable old musicals like Show Boat and West Side Story,If/Then is gaming up for its Broadway opening next March with a run in sleepy Washington, D.C., at the National Theatre.

Despite its pedigree—If/Then is the work of Tom Kitt and Brian Yorkey, the writers of the Pulitzer Prize– and Tony Award–winning Next to Normal, and Michael Greif, the director of the Pulitzer Prize– and Tony Award–winning Rent—this overlong, undermotivated beta version shuffles along, snapping every so often into sharp melodic focus while remaining narratively inert. To stick with (and perhaps abuse) the subway metaphor, it’s a hot, crowded local, not an express.

In once sentence: The M.C. Escher–like latticework stage (designed by Mark Wendland) is terrific; the songs are, with a couple of exceptions, so-so; and the story is limp. Book and lyric writer Yorkey has cribbed not just his show’s binary structure from the Gwyneth Paltrow vehicle Sliding Doors, but several plot points, too.

But the problems here go deeper than just a more-than-casual resemblance to a 15-year-old movie—or the superficiality of its much-discussed urban planning element. The musical doesn’t just trade the charisma of its star, Idina Menzel. It positively depends upon it to conceal the fact that the heroine that Yorkey has given her to play is as fundamentally passive as Forrest Gump, even after the story gives us two of her to follow along separate, parallel paths. Yorkey want to explore the “butterfly effect” idea that the smallest decisions—in this case, how long each iteration of Elizabeth chooses to linger in Madison Square Park—can have life-altering consequences that we can’t possibly be cognizant of at the time. 

But again and again and again, Elizabeth, in both her “Liz” and “Beth” identities, is offered prizes—jobs, sex partners, devoted friends—she has expended no effort to get. This no-pursuit, all-reward dynamic gives the show a listless quality that it never manages to shake off.

Now 42, Menzel is a powerhouse singer and solid comedienne who originated lead roles in Rent and Wicked, both huge hits. Here she plays Elizabeth, a city planner in her late 30s (but why can’t she be 42?) returning to Manhattan after a dozen unhappily married years of exile in Phoenix. Phoenix comes in for a lot of abuse. Nebraska gets off relatively easy, once she starts seeing a guy from Nebraska. There isn’t actually a song here called “I’m New York City and You’re Not,” but the attitude pervades.

For all the show’s bravado, its primary character is recognizably instantly as a shopworn cliché from city-planning central casting: She’s hyper-competent and clinically humorless, like Mr. Spock in a Banana Republic blazer. When someone makes an anecdotal remark about the likelihood of this or that, Elizabeth can’t stop herself from turning it into an equation. But you can’t quantify sex, to her out-loud dismay. She tries, bless her heart, inviting a guy into her apartment for “75 to 80 minutes.” It's less funny than it sounds.

One of Elizabeth’s selves lucks into a plum job as the city’s deputy director of urban planning, working for an old pal who adores her. That would be Jerry Dixon, who sings beautifully and has the rare honor of playing a fully-developed character. This version—Liz? Beth? Despite lighting designer Kenneth Posner’s use of a different palette for each story, I lost track of which Elizabeth was which pretty quickly—ends up messing around noncommittally with Lucas, her bisexual boyfriend from college, now a smart-growth activist who must sell his blood and semen to make ends meet. Adam Rapp, Menzel’s costar from Rent, ably fills the part. Like everyone else, Lucas worships Elizabeth despite her indifference. He even gets a plaintive song about it, “You Don’t Need to Love Me,” and it’s one of the show’s best. She responds to his affections by using them to get him to drop his group’s opposition to “the far West Side project.”

Meanwhile, her doppelganger—Beth?—takes a less stimulating job as an adjunct professor of urban planning. She’s drawn into a relationship with Josh (James Snyder), an at once unthreatening and unaccountably persistent Army reservist trauma surgeon just returned from a tour of duty overseas.

In a show that seems so proud of its alleged modernity, the notion that a healthy single dude just back from a war zone where—as he points out himself—he witnessed death on a daily basis would bother to keep pursuing a grim woman who repeatedly turns him down without showing the slightest flicker of interest seems awfully quaint. (Maybe he’s seeing other people.) Snyder is handsome and can certainly sing, but his character, too, remains a cipher. And anyway, after escaping a decade-plus of crappy marriage, is boring-but-stable Josh what Liz, or Beth, or whomever, would be looking for in a man?

I can’t answer that question. Yorkey and composer Kitt seem not to know what their protagonists want—the most important question in any story. In its best moments, the show harvests this confusion: Menzel’s song “WTF?” finds her sing-talking both her selves into sleeping with the dude waiting in her living room. Only they’re different dudes. It’s clever, but that, too, poses a problem: The need for most of Menzel’s songs to comprise a Beth half and a Liz half makes the 165-minute show (including intermission) feel even longer than it is.

Those who’re showing up just to hear Menzel sing will get their money’s worth regardless, even given the hefty (though still sub-Broadway) freight of the ticket. Punters attracted to the city planning aspect of the show will feel less nourished: At the end of the (very long) day, If/Then doesn’t have any more to say about urban planning than Spider-Man (in its stage musical, film, or comic book iterations) does about photojournalism. It’s just the writers’ idea of the sort of profession that their brilliant and dynamic heroine would be drawn to. But even when they attempt to demonstrate Elizabeth’s skill, they show her as skilled at playing politics, not at the actual nuts and bolts, as it were, of a job designing cities.

The show comes to life in its supporting characters: Dixon, as well as LaChanze and Jenn Colella, playing the on-again, off-again lesbian couple Kate and Anne, actually register as people, not satellites that exist only to orbit Liz/Beth and remark incessantly on how extraordinary she is. You can get away with that, sometimes, for as long as your star sticks around. But a rewrite would be a better strategy. As things stand, if Mendel leaves, then Yorkey and Kitt will have a big black hole where a heroine should be.